1. Field of the Invention
The present invention relates to a multicolor electrophotographic apparatus using an intermediate transfer belt.
2. Description of the Related Art
In association with a recent trend toward colorization and expedition of documents, faster color laser beam printers have been pursued rapidly.
A tandem color electrophotographic apparatus is mentioned as an example of a color printer. In this printer, toner of black (K), yellow (Y), magenta (M), and cyan (C) colors is used, and image forming means of respective colors are provided. Toner images formed on the respective image forming means are transferred to an intermediate transfer belt in a superimposing manner, to thus form a color image.
As shown in FIGS. 1A and 1B, in the electrophotographic apparatus adopting the tandem mechanism, a plurality of image forming means 102 to 105 independently and sequentially form different toner images on respective photosensitive drums, and the thus-formed toner images are transferred on an intermediate transfer belt 101 in a superimposing manner. Therefore, the electrophotographic apparatus of this type easily addresses a speedup in printing speed. However, a position where the toner image formed by the image forming means is to be transferred on the intermediate transfer belt 101 is easily displaced, and the electrophotographic apparatus involves a problem of occurrence of a phenomenon of registration offset (hereinafter called “registration offset”), that is, a phenomenon of offsets arising among the positions where the toner images of colors are to be superimposed on each other.
Among reasons for the registration offset, an initial reason is manufacturing tolerances of the respective image forming means and mount tolerances of the same; and a time-varying reason is thermal expansion or deformation of members attributable to changes in the internal temperature of the electrophotographic apparatus. When laser is used as exposure means to be provided within the image forming means, the phase of a polygon mirror varies from one image forming means to another, which is in turn responsible for a registration offset.
For instance, as shown in FIG. 2, the following technique has already been put forth as means for preventing occurrence of a registration offset. Specifically, registration patches 201, which are toner images to be used for controlling correction of registration, are formed in respective colors on the intermediate transfer belt 101. The amount of registration offset between toner images of respective colors is detected by means of registration patch detectors 112. On the basis of a detection result, exposure means corrects a write timing of a laser beam for forming a latent image on each image forming device and controls the phase of a polygon mirror.
A technique using photoelectric elements as position detection means has been proposed. This technique uses two photoelectric elements for detecting the position of a light beam; specifically, as shown in FIG. 4A, photoelectric elements 401, 402 are arranged. When the light beam has passed so as to cross the photoelectric elements 401, 402, light currents IPD1, IPD2 are output from the photoelectric elements 401, 402 while phases of the currents remain offset, as shown in FIG. 4B. At this time, even when fluctuations have arisen in the quantity of light of the light beam, a cross point between the light currents IPD1, IPD2 does not fluctuate and is held constant. However, when the light beam has not yet entered the photoelectric elements, light current values of the light currents IPD1, IPD2 remain close to each other, which induces faulty operation. As shown in FIG. 4C, when light does not enter the photoelectric elements, an output from one of the photoelectric elements is clamped to a predetermined voltage, whereby a difference arises between this output and an output from the other photoelectric element. As a result, detection of a cross point is performed such that Vo is output at only a point where the outputs from the two photoelectric elements 401, 402 cross each other, as shown in FIG. 4D (Japanese Patent No. 3,068,865).
However, under circumstances where positions are detected by means of the light reflected from the intermediate transfer belt and the light reflected from the registration patches formed thereon, when the photoelectric elements 401, 402 are arranged as shown in FIG. 10A and the patches formed on the intermediate transfer belt have crossed the photoelectric elements 401, 402, the light currents IPD1, IPD2 are output from the photoelectric elements 401, 402 while being out of phase with each other, by means of the light reflected from the intermediate transfer belt and the patches, such as that shown in FIG. 10B1. At this time, even when fluctuations have arisen in the quantity of reflected light, the cross point between the light currents IPD1, IPD2 does not fluctuate and is held constant. However, when the light reflected from the intermediate transfer belt falls on the photoelectric elements, the light current values of the light currents IPD1, IPD2 are close to each other, which is in turn responsible for faulty operation. Therefore, as shown in FIG. 10C1, when the light reflected from the intermediate transfer belt does not fall on the photoelectric elements, an output from one of the photoelectric elements is clamped to a predetermined voltage, whereby a difference arises between this output and the output from the other photoelectric element. Consequently, detection of a cross point can be performed only at a point where the outputs from the two photoelectric elements cross each other, as shown in FIG. 10D1. However, under circumstances where changes have arisen in the quantity of light reflected from the intermediate transfer belt for reasons of a characteristic of the intermediate transfer belt, fluctuations in the quantity of light of the light source, or the like and where the quantity of reflected light has increased as shown in FIG. 10B2, when the output is clamped to the predetermined voltage as shown in FIG. 10C2, the cross point between the light currents IPD1, IPD2 exists in a voltage range higher than the clamp voltage. As a result, a cross point is detected at a point differing from the cross point between the light currents IPD1, IPD2, as shown in FIG. 10D2. When the quantity of reflected light has decreased as a result of changes having arisen in the quantity of light reflected from the intermediate transfer belt for reasons of the characteristic of the intermediate transfer belt or fluctuations in the quantity of light of the light source, as shown in FIG. 10B3, and when the output is clamped at the predetermined voltage as shown in FIG. 10C3, the cross point between the light currents IPDI, IPD2 exists in a voltage range equal to or lower than the clamp voltage, and hence the light currents IPD1, IPD2 come close to each other, which is in turn responsible for faulty operation. Detection of a cross point is performed at the cross point between the light currents IPDI, IPD2 and at the intermediate transfer belt section, as shown in FIG. 10D2, which raises a problem of a failure to detect the cross point between the outputs of the two photoelectric elements.
There has also been put forward a technique for detecting the amount of offset by means of a CCD sensor employed as another means for detecting the amount of offset. This technique is to take toner images having different reflection characteristics as base materials at the time of formation of registration patches and to detect black patches which have no reflection areas and are formed on the base materials. However, patches of different colors are formed as base materials for detecting the black patches, which raises another problem of an increase in the quantity of toner consumption (see Japanese Patent No. 2,761,287).